1. Field of the Invention
The invention relates to a process for dechroming, and/or depleting the MgO content of, steel slags.
2. Description of Related Art
Slags used as starting products or fluxes in the production of cement have already been proposed several times. From U.S. Pat. No. 4,174,974 it is, for instance, known to react coal slag in the molten state with mineral substances and, in particular, calcium oxide, calcium hydroxide or calcium carbonate in order to produce clinker or cement in that manner. In the same U.S. Pat. No. 4,174,974, the use of blast furnace slag for comparable purposes has already been described. The admixture of slag into a rotary tubular kiln charged with kaolin, bauxite and lime has also been suggested.
Generally, the extensive use of steel slags as fluxes in the production of clinker and cement, is in conflict with the high content of metals different from iron contained in such steel slags. With the increasing use of scrap as a steel production prematerial, the portion of alloying metals in the steel slag has risen. In particular, a high content of chromium largely excludes the use of such slags as raw materials or fluxes. Due to the high toxicity of CrVI salts, only extremely slight amounts of CrVI are tolerated for construction materials.
Another disadvantage of the use of steel slags as fluxes in the production of clinker and cement resides in a usually high MgO content, which may amount to 10 to 15% by weight of the slag. In the hydration of cement, MgO is converted to Mg(OH)2 (brucite), whereby the compressive strength is strongly reduced on account of a volume expansion. For that reason, it is usually anticipated that the MgO content in clinker should not exceed 6% by weight.